Steam burning fuel?? Increasing performance

A mechanic suggested me installing a system (a conduit connecting some point and the fuel duct.... dont know how to mention.... it increases the sensitivity by introducing some steam into the fuel.... something like that)

After installing it, the response of the engine will be much faster.

The price is HKD2500 (USD320)

My questions are

1.) do u know what it is? i mean the correct term in English?
2.) any harm to the engine (he said no of course)
3.) increasing power and torque? (he said yes of course)
4.) reducing fuel? (he said yes of course)

Thanks!